1. Field of the Invention
The present invention relates to a variable gain amplifier (VGA) and more particularly to a high linearity gain distributed variable gain amplifier which provides enhanced performance in both RF and digital communication systems.
2. Description of the Prior Art
Wideband variable gain amplifiers (VGA) with high linearity are used in various signal control applications, such as instrumentation RF communications and lightwave opto-electronic communication systems. In such communication systems, it is desirable to have a high linearity VGA for receiver applications in order to obtain a relatively high dynamic range. In RF communications, it is desirable that the VGA have good linearity and low intermodulation distortion at the VGA's low gate gain setting when the incident signal is at a peak in power level during reception. In digital light wave communication applications, it is desirable to maintain good phase and amplitude linearity and broad bandwidth, especially in the presence of a high power signal in order to maintain good reception. In these systems, it is desirable that the VGA cover a wide frequency range from dc to about 3 times the highest bit rate in order to ease its architectural implementation and maintain good receiver sensitivity and bit-error rate (BER).
In digital communication systems, and specifically receiver applications, there is a need for VGAs with high linearity, wide bandwidth and direct coupled capability. A VGA in such an application must have the following characteristics: low intermodulation distortion and excellent phase and amplitude linearity over a relatively wide gain control range especially at low gain settings; direct coupled capability (i.e. no dc blocking capacitors in the RF path) allowing frequency performance down to dc; and high upper end bandwidth response.
VGAs are known in the art. An example of a VGA is disclosed in "A Monolithic Wideband Variable Gain Amplifier With a High Gain Range and Low Distortion", by P. van Lieshout et al. IEEE International Solid State Circuits Conference Digest, February, 1997, pp. 358-359. However, the practical employment of the VGA Gm gain control technique described in the above-mentioned reference is not amenable to the required distributed amplifier RF performance for several reasons. Firstly, the disclosed Gm gain control technique uses a resistive ladder to create the distributed dc offset voltages for the parallel connected differential amplifiers for the different distributed amplifier sections. This resistive ladder is in series with the RF input path and thus is nonattractive with a distributed amplifier topology because the series resistive ladder produces resistive transmission line attenuation that is cumulative with each additional section of distributed amplifier, thus causing decreasing benefits with each successive stage. In addition, the restrictive ladder limits the practical number of stages or sections that can be used and therefore limits the total gain control, which is a function of the number of sections and differential amplifier stages. Secondly, the Gm gain control disclosed in the above mentioned reference requires a current switching network for employing variable (tunable) dc voltage offset for the variable Gm gain control. This current switching network is typically formed from a differential current source switch which loads down the input synthetic transmission lines of the distributed amplifier with its high impedance collectors and thus causes instability as well as limited frequency bandwidth performance.
Other VGA amplifiers are known, for example, as disclosed in "A Broadband Local Noise Dual Gate FET Distributed Amplifier" by W. Thompson, 1989 IEEE MMWMC-S Digest, pp. 11-14. The variable gain amplifier disclosed in this reference utilizes a dual gate FET. However, such dual gate VGAs present a major disadvantage. In particular, dual gate FETs operated as VGAs are known to not maintain the linearity characteristics at low gain states and in fact actually get worse at low gain settings. Thus, there is a need for a high linearity variable gain distributed amplifier which can provide enhanced performance in various systems including RF and communication systems.